This thesis focuses on protoplanetary disks: flattened structures of gas and dust around young stars in which planets are expected to form and grow. Physical-chemical models that compute the... Show moreThis thesis focuses on protoplanetary disks: flattened structures of gas and dust around young stars in which planets are expected to form and grow. Physical-chemical models that compute the thermal structure and chemical composition of protoplanetary disks are compared to observations to increase our understanding of the processes that shape these disks.Chapters two and three investigate the sizes of protoplanetary disks in the context of evolution of the dust. A gas disk that is observed to be four times more extended than the dust disk is found to be a clear indication that the dust has drifted inward. Detailed modeling reveals that five out of a sample of 10 disks in the Lupus star-forming region show evidence for dust evolution. Chapter four shows that observed gas outer radii are consistent with disks evolving viscously, assuming disks start out small and evolve slowly. Chapter five reveals that the chemical conversion of CO into more complex species cannot by itself explain the low observed CO isotopolog line fluxes. Finally, Chapter six uses non-detections of the HD emission line to put an upper limit on the total mass of disks and rules out that they are currently gravitationally unstable. Show less
This thesis discusses the physical and chemical processes than influence the composition of forming planets. The focus is on the effect of these processes on the abundance and distribution of... Show moreThis thesis discusses the physical and chemical processes than influence the composition of forming planets. The focus is on the effect of these processes on the abundance and distribution of moleculus in the birth places of planets, proto-planetary disks. The thesis further discusses how current and future observations can be used to quantify the effects of these processes. A good grasp of these processes will enable researchers to link the composition of (exo)planets to their formation origin. Specifically this thesis studies the the most abundant carbon and oxygen carrying molecules, CO, CO2 and H2O. Chemical modelling of CO indicates that conversion of CO into other molecules needs to be fast to explain the current observations. Study of the transport and distribution of CO2 indicate that the measured the CO2 abundance in the surface layers of disks is not bulk CO2 abundance. The power of spectroscopic CO observations in the infrared to find physical structures in disks, possibly created by planets is also discussed. Finally, a combination of infrared observations is used to show that the closest proto-planetary disk is poor in molecules that contain carbon and oxygen, indicating that a process is removing these molecules from the gas phase. Show less
The SimpleX radiative transfer method is based on the interpretation of photons as particles interacting on a natural scale: the local mean free path. In our method, light is transported along the... Show moreThe SimpleX radiative transfer method is based on the interpretation of photons as particles interacting on a natural scale: the local mean free path. In our method, light is transported along the lines of an unstructured Delaunay mesh that encodes this natural distance and represents the physical medium. The SimpleX method is fast, highly adaptive and its computational cost does not scale with the number of sources. It is therefore well-suited for cosmological applications where it is essential to cover many orders of magnitude in resolution and where millions of sources can exist within a single simulation. We describe the code, its recent developments and apply it to several relevant astrophysical problems. In particular, we perform radiative transfer calculations of cosmological reionization and of the wind-wind interaction region of the Eta Carinae binary system. Show less
The appearance of the first luminous sources in the Universe marks the beginning of the epoch of reionisation. The cosmic reionisation of hydrogen has been the last major phase transition in the... Show moreThe appearance of the first luminous sources in the Universe marks the beginning of the epoch of reionisation. The cosmic reionisation of hydrogen has been the last major phase transition in the evolution of the Universe, turning cool neutral gas into the hot, ionised plasma we observe today. Up to now, direct observations of reionisation have been impossible, most of our current knowledge therefore stems from numerical studies. However, even with current computing facilities the modelling of the reionisation epoch presents some serious challenges. Radiative transfer is perhaps the biggest challenge, due to the high dimensionality and non-locality of the problem. In this thesis the latest improvements of the SimpleX method, that was designed to overcome some of the problems related to radiative transfer, are presented. We have used this method to study the escape fraction of ionising photons from galaxies. Our results show that high-redshift dwarf galaxies can be important contributors to cosmic reionisation. In addition, we have studied the effect of the inclusion of diffuse recombination radiation in reionisation simulations on large scales, which has been neglected by recent studies. We show that diffuse recombination radiation does not have a significant influence on the reionisation process. Show less
The first stars formed a few hundred million years after the Big Bang, when the Universe was only a small fraction of its present age. Their radiation transformed the previously cold and neutral... Show moreThe first stars formed a few hundred million years after the Big Bang, when the Universe was only a small fraction of its present age. Their radiation transformed the previously cold and neutral hydrogen that filled intergalactic space into the hot and ionised cosmic plasma that is observed today. This milestone in the history of the Universe is called the epoch of reionisation. Much about reionisation is still unknown. Computer simulations are one of the most promising theoretical tools to study reionisation. The wealth of high-quality data that will soon be provided by the next generation of telescopes, specifically designed to observe the reionisation event, make it a particularly exciting time to perform such simulations. The thesis "Simulating Cosmic Reionisation" presents TRAPHIC, a novel method to include the transport of ionising radiation emitted by the first stars in simulations of reionisation. TRAPHIC (TRAnsport of PHotons In Cones) is one of the first of a new type of radiative transfer methods that allow the accurate and efficient computation of the growth of ionised regions in representative models of the Universe that contain hundreds of millions of stars. First simulations that employ TRAPHIC on the Dutch national supercomputer Huygens demonstrate the importance of the concepts that underly its design. Show less
Dark matter assembles to form massive halos, attracting gas particles that coalesce to form stars, and the photons produced by these sources are sent into the void. An accurate description of the... Show moreDark matter assembles to form massive halos, attracting gas particles that coalesce to form stars, and the photons produced by these sources are sent into the void. An accurate description of the transport of particles is therefore the essential ingredient for understanding how the Universe evolved into its present form. Of all these transport processes, the transfer of radiation is the most elusive: the non- locality and high-dimensionality of the problem currently put realistic simulations out of reach. In this thesis, we present a novel approach to this transport problem, combining methods from mathematics and statistical physics in a practical computer code. Its speed and versatility allow us, for the first time, to make realistic simulations in radiative gas dynamics and in cosmology. Because of the general mathematical/physical character of our methods, we expect them to be useful for transport phenomena in general. Show less
The late stages of evolution of stars like our Sun are dominated by several episodes of violent mass loss. Space based observations of the resulting objects, known as Planetary Nebulae, show a... Show moreThe late stages of evolution of stars like our Sun are dominated by several episodes of violent mass loss. Space based observations of the resulting objects, known as Planetary Nebulae, show a bewildering array of highly symmetric shapes. The interplay between gasdynamics and radiative processes determines the morphological outcome of these objects, and numerical models for astrophysical gasdynamics have to incorporate these effects. This thesis presents new numerical techniques for carrying out high-resolution three-dimensional radiation hydrodynamical simulations. Such calculations require parallelization of computer codes, and the use of state-of-the-art supercomputer technology. Numerical models in the context of the shaping of Planetary Nebulae are presented, providing insight into their origin and fate. Show less
